The present invention relates to detection of signals on communications channels. More particularly, the present invention relates to marking unreliable symbols read back from a hard disk drive.
Data stored on magnetic media, such as hard disk drives, are typically encoded using an error-correction code, such as a Reed-Solomon code, so that errors that occur during storage and read back of the data can be detected and corrected. In conventional systems, the read back signal is typically detected by a channel detector, such as a Viterbi detector, that generates multi-bit symbols based on the read back signal. These symbols often include errors due to imperfect detection and noise.
Conventional systems typically include an error-correction decoder, such as a Reed-Solomon decoder, that uses the error-correction code in the data to correct these errors. Each error-correction code, and its associated decoder, has a power t, and can correct t symbol errors in the read back data sequence. When a sequence has more than t symbol errors, the decoder cannot correct the symbol, and typically triggers a retry, in which the data is read back from the disk a second time, detected again, and decoded again. This process can repeat until the detected sequence has t or fewer symbol errors, in which case it can be corrected by the decoder. Of course, this repetitive retry process consumes significant amounts of time, thus reducing the rate at which data can be read back from the hard disk drive.